Exception register delay

ABSTRACT

A processor includes: memory; an execution pipeline having a plurality of pipeline stages configured to process data provided to the execution pipeline and to store a result of the processing into the memory; a receive pipeline having a plurality of pipeline stages configured to handle incoming data to the processor and storing the incoming data into memory; context status storage configured to hold an exception indicator of an exception encountered by the execution pipeline while the execution pipeline processes data; wherein the receive pipeline is configured to determine that an exception has been committed to the context status storage by the execution pipeline, to suppress a write to memory of any incoming data to be handled by the receive pipeline and to commit a corresponding exception indicator to the context status storage at a final one of its pipeline stages.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application claims priority to United Kingdom's PatentApplication No. 2007910.9, filed on May 27, 2020, the disclosure ofwhich is hereby incorporated herein by reference in its entirety.

TECHNICAL FIELD

The present disclosure relates to the handling of exceptions in aprocessing unit having an execution pipeline.

BACKGROUND

There has been an increased focus on the development of computers whichare particularly suitable for executing machine intelligence graphs.Machine intelligence graphs consist of many nodes connected to oneanother via edges, when nodes represent a certain processing requirementon their individual data and edges represent data to be transferredbetween nodes. This structure allows for a high level of parallelism tobe performed, and there have been developments in computer architecturesto support such high level of parallelism.

Once such architecture has been developed by Graphcore and is describedfor example in GB applications Nos. 1717295.8; 1717304.8; 1717299.0, thecontents of which are herein incorporated by reference. The computerdeveloped by Graphcore comprises an arrangement of multiple tiles on thesame chip (i.e. same die), each tile comprising its own separaterespective processing pipeline and memory (including program memory anddata memory). Separate portions of program code can be run in parallelon different ones of the tiles. The tiles are connected together via anon-chip interconnect which enables the code run on the different tilesto communicate. That is, data may be exchanged over the on-chipinterconnect. Data may be transmitted from one tile to another tile toact as an input for a process on the other tile. The data memory may beutilised by the in-tile processing pipeline for load and storeoperations accomplished by the pipeline. In addition, the data memorycan be used to store data which is incoming to the tile from anothertile on the same chip. Furthermore, the data memory can be used to storedata incoming from sources external of the chip. The chip has anexternal exchange mechanism which allows it to receive data fromexternally of the chip and to provide that data to one or more tiles onthe chip.

The processing pipeline on each tile may itself run multiple concurrentthreads on the tile. In that case, a scheduler can be provided on eachtile to support interleaving of multiple threads on the same tilethrough the same execution pipeline.

When a computer is operating it can encounter exceptions. Exceptionsindicate unusual and potentially problematic execution conditions, thatis a divergence from the normal execution behaviour of a program. Whenan exception is encountered, it is expected that the computer will havethe ability to raise a signal which indicates externally of the chipthat an exception has been encountered. This enables a user to establishthat something has gone wrong in the behaviour of the computer. Thereare many possible causes of exceptions. What is important is to be ableto ascertain the cause of an exception, and in some cases to enable acomputer to recover from the exceptions.

It is important when examining the cause of an exception to be able tohave access to the state of the computer at the time that the exceptionoccurred. This can be a particular challenge when dealing withexceptions in execution pipelines.

It is also important for verification of a processor that the state ofthe processor around exceptions can be modelled, in order that exceptionbehaviour can be tested.

SUMMARY

The present inventor has recognised that there are particular challengesto be met in the context of a processing unit which comprises twopipelines which may operate independently. In the Graphcore computerreferenced in the background, multiple tiles may exchange data betweenthem. Data to be exchanged may be generated by tiles on the chip or maybe received from a source externally of the chip. Receive logic on eachtile handles the incoming data to the tile, whether it arrives from aninternal (on-chip) source or from an external (off chip) source. Wherethe receive logic itself is pipelined, it is challenging to ensure thatany exception which is raised is correctly handled within the tile, andalso that the state of the tile at the time at which the exception wasraised is correctly available for subsequent inspection. Moreparticularly, the present disclosure addresses the challenge of handlingincoming data by the receive logic when an exception is encountered inan instruction stream being executed in the tile.

One aspect of the invention provides a processor comprising:

-   -   memory;    -   an execution pipeline having a plurality of pipeline stages for        processing data provided to the execution pipeline and storing a        result of the processing into the memory;    -   a receive pipeline having a plurality of pipeline stages for        handling incoming data to the processor and storing the incoming        data into memory;    -   context status storage for holding an exception indicator of an        exception encountered by the execution pipeline whilst it is        processing data;    -   the receive pipeline being configured to determine that an        exception has been committed to the context status storage by        the execution pipeline, to suppress a write to memory of any        incoming data to be handled by the receive pipeline and to delay        committing a corresponding exception indicator to the context        status storage until a final one of its pipeline stages.

In some embodiments the execution pipeline is configured to check thecontext status register when retiring an instruction and on detection ofthe corresponding exception indicator in the context status storage tocause an exception signal to be raised.

In some embodiments, the receive pipeline is configured to process afirst incoming data word in sequential pipeline stages, and to processone or more subsequent incoming data words in successive ones of thepipeline stages, whereby multiple pipeline stages are operating onsuccessive data words at the same time.

In some embodiments the pipeline stages of the receive pipeline aretimed according to a clock signal. The pipeline stages of the executionpipeline may also be timed according to a clock signal. Where the clocksignals are the same frequency (or the same clock signal) the exceptionhandling behaviour can be consistent between execution pipeline andreceive pipeline exceptions.

In some embodiments the receive pipeline is configured to determine in afirst one of the pipeline stages that the exception has been committed.

In some embodiments, the execution pipeline is configured to process insuccessive pipeline stages incoming data comprising data words, and tocarry out a result storing operation for each incoming data word whereinthe number of result operations available to store any result to memorybetween detection of the exception and commitment of the exceptionindicator correspond to one less than a number of pipeline stages in theexecution pipeline. It will be appreciated that there may not be a dataword to be processed at each pipeline cycle.

In some embodiments the execution pipeline and the receive pipelineoperate contemporaneously, the processor comprising access logic forresolving memory write conflicts to the memory of data from theexecution pipeline and the receive pipeline.

The access logic may be configured to prioritise a memory write of datafrom the receive pipeline over data from the execution pipeline in theevent of a memory access conflict.

The processor may be configured to set an error value when a write tomemory is detected after an exception has been committed.

Another aspect of the invention provides a method of handling exceptionsin a processor comprising:

-   -   memory;    -   an execution pipeline having a plurality of pipeline stages for        processing data provided to the execution pipeline and storing a        result of the processing into the memory;    -   a receive pipeline having a plurality of pipeline stages for        handling incoming data to the processor and storing the incoming        data into memory; and    -   context status storage for holding an exception indicator of an        exception encountered by the execution pipeline whilst it is        processing data, the method comprising:    -   determining by the receive pipeline that an exception has been        committed by the execution pipeline;        suppressing a write to memory of any incoming data to be handled        by the receive pipeline; and    -   committing a corresponding exception indicator to the context        status storage after a delay until a final one of the pipeline        stages of the receive pipeline.

In some embodiments, the execution pipeline checks the context statusregister when retiring an instruction and on detection of thecorresponding exception indicator in the context status storage causesan exception signal to be raised.

In some embodiments, the receive pipeline processes a first incomingdata word in sequential pipeline stages, and processes one or moresubsequent incoming data words in successive ones of the pipelinestages, whereby multiple pipeline stages are operating on successivedata words at the same time.

The pipeline stages of the receive pipeline and the execution pipelinemay be timed according to a clock signal.

In some embodiments, the receive pipeline detects the exceptionindicator in the first one of the pipeline stages.

In some embodiments the data processed by the pipeline comprisessuccessive data words, and the number of data words which are storedinto memory between detection of the exception by the execution pipelineand commitment of the exception indicator correspond to one less than anumber of pipeline stages in the execution pipeline.

In some embodiments, the execution pipeline and the receive pipelineoperate contemporaneously, the processor comprising access logic forresolving memory write conflicts to the memory of data from theexecution pipeline and the receive pipeline.

The access logic may be configured to prioritise a memory write of datafrom the receive pipeline over data from the execution pipeline in theevent of a memory access conflict.

BRIEF DESCRIPTION OF THE DRAWINGS

For a better understanding of the present invention, and to show how thesame may be carried into effect, reference will now be made by way ofexample to the accompanying drawings.

FIG. 1 is a schematic view of a processing unit;

FIG. 2 is a schematic view of an execution pipeline;

FIG. 3 is a schematic view of a receive logic pipeline;

FIG. 4 is a timing diagram showing when a retirement exception iscommitted;

FIG. 5 is a timing diagram showing when a receive exception is committedto a context register of the processing unit;

FIG. 6 is a timing diagram showing when an execution pipeline exceptionis committed to a context register;

FIG. 7 is a schematic diagram showing context registers;

FIG. 8 is a schematic diagram of a chip with multiple tiles;

FIG. 9A is a flow diagram showing behaviour of the receive pipeline whenencountering receive exceptions;

FIG. 9B is a flow diagram showing behaviour of the receive pipeline whenencountering tile exceptions; and

FIG. 10 is a schematic view of memory, showing its access ports.

DETAILED DESCRIPTION

The present disclosure concerns exceptions which arise during operationof a processing unit. Some exceptions are “fatal”. That is, theprocessing unit is not expected to recover itself from the exception.What is important is that the exception is flagged externally of theprocessing unit, for example to a debug tool or other inspection toolsuch that the cause of the exception can be investigated to prevent thesame problem occurring again. The present disclosure applies toexceptions which arise in normal operation and/or in a debug context.

To enable the cause of exceptions to be determined, state associatedwith the exceptions is recorded. This state comprises the state at whichthe processing unit had reached when the exception was raised, and mayinclude additional state indicative of the exception itself in exceptionregisters discussed later.

In inspecting and debugging exceptions, it is important that theunderlying state of the processing unit at the time at which theexception was raised provides useful and consistent information. Thisprovides a number of complex challenges.

These are discussed further herein with the solutions offered by thepresent disclosure.

Firstly, by way of non-limiting example, an example of a processormodule 4 is described with reference FIG. 1. The processor module 4 maybe one tile of an array of like processor tiles on the same chip, or maybe implemented as a standalone processor on its own chip. The processormodule 4 in this example comprises a processing unit 10 and a localmemory 11 (i.e. on the same tile in the case of a multi-tile array, orsame chip in the case of a single processor chip). The memory 11comprises an instruction memory 12 and the data memory 22 (which may beimplemented in different addressable memory units or different regionsof the same addressable memory unit). The instruction memory 12 storesmachine code instructions to be executed by the processing unit 10. Thedata memory 22 stores both data to be operated on by the executed code,and data output by the executed code (e.g. as a result of suchoperations). In addition, the data memory may receive data written tothe tile from a source outside the tile via receive logic at a receiveinterface 17.

The memory 12 stores a variety of different threads of a program, eachthread comprising a respective sequence of instructions for performing acertain task or tasks. Note that an instruction as referred to hereinmeans a machine code instruction, i.e. an instance of one of thefundamental instructions of the processor instruction set, consisting ofa single opcode and zero or more operands. Note that operands areindicated by instructions. They may be immediate and encoded by acompiler, or accessed from registers. Data in the registers may havebeen loaded into the registers from the data memory 22. This could bedata which has been stored as a result of previous operations by thetile, or data incoming from an external data source via the receiveinterface 17.

Within the processing unit 10, multiple different threads from theinstruction memory 12 can be interleaved through a single executionpipeline 13 (though, typically, only a subset of the total threadsstored in the instruction memory can be interleaved at any given pointin the overall program). The multithreaded processing unit 10 comprises:

-   -   a plurality of context register files 26, each arranged to        represent the state (context) of a different respective one of        the threads to be executed concurrently;    -   a shared processing pipeline 13 that is common to the        concurrently executed threads; and    -   a scheduler 24 for scheduling the concurrent threads for        execution through the shared pipeline in an interleaved manner,        preferably in a round robin manner.

The processing unit 10 is connected to the shared instruction memory 12common to the plurality of threads and the shared data memory 22 that isagain common to the plurality of threads.

The processing pipeline 13 comprises a fetch unit 14, a decode unit 16and an execution unit 18 comprising an execution pipeline. The executionpipeline will be described in more detail later, and it may performarithmetical and logical operations, address calculations, load andstore operations and other operations s defined by the instruction setarchitecture. Each of the context register files 26 comprises arespective set of registers for representing the program state (context)of a respective thread. The context register files include contextregisters where exception status for that thread may be stored. Thecontext register files also include registers for holding exceptionstatus for the receive logic at receive interface 17. In someembodiments, the threads may be worker threads that are scheduled ormanaged by a supervisor thread. The supervisor thread performs controlfunctions for the tile, and may additionally process its own incomingdata to the tile. The supervisor thread may be associated with its owncontext registers which include registers for holding exception statusof the receive logic at receive interface 17. These are discussed inmore detail later.

The fetch unit 14 is connected so as to fetch instructions to beexecuted from the instruction memory 12 under control of the scheduler24. The scheduler 24 is configured to control the fetch stage to fetchan instructions from each of a set of concurringly executing threads inturn in a repeating sequence of timeslots, thus dividing the resourcesof the pipeline 13 into a plurality of temporarily interleavedtimeslots. For example, the scheduling scheme could be round robin orweighted round robin.

The fetch stage 14 has access to a program counter (PC) of each of thecontexts. For each respective thread, the fetch unit 14 fetches the nextinstruction of that thread from the next address in the program memory12 as indicated by the program counter. The program counter incrementseach execution cycle and less branch by a branch instruction. The fetchunit 14 then passes the fetched instruction to the decode stage 16 to bedecoded, and the decode unit 16 then passes an indication of the decodedinstruction to the execution unit 18 along with the decoded addresses ofany operand registers specified in the instruction, in order for theinstruction to be executed. The execution unit 18 has access to theoperand registers and two control registers which it may use inexecuting the instruction based on the decoded register addresses. Ifthe instruction defines a memory access (load or store) load store logicof the execution unit 18 (to be described later) loads a value from thedata memory 22 into an operand register of the respective thread, orstores a value from an operand register of the respective thread intothe data memory 22, in accordance with the instruction.

Items can be written into the memory 11 from two different sources. Afirst source is the execution pipeline which processes data incoming toit according to instructions which have been fetched from theinstruction memory and writes back processed data into the memory. Asecond source that can write data to the memory is receive logic 17 atthe receive interface of the processing unit. This receive logic mayitself be pipelined, and is sometimes referred to herein as the receivepipeline. The receive logic 17 receives data incoming to the processingunit from an external source. Possible external sources of data into theprocessing unit are discussed later. Exceptions may be raised byprocessing operations in the execution pipeline and/or in the receivelogic.

The receive pipeline and the execution pipeline are controlled by aclock signal with the same frequency—that is with the same delay betweenpipeline stages. This could be the same clock, or clock signals havingthe same frequency from a common clock source, derived from a processorclock. An example clock 3 is shown in FIG. 8, but for reasons of clarityits wires connecting it to the pipelines are not shown. Note howeverthat operation of the pipelines may be asynchronous.

FIG. 2 shows the stages of the execution pipeline 18. These executionstages define where the functional behaviour of an instruction occurs. Aprior stage f4/i0 represents decode and issue of instructions into thepipeline 18. Address generation and memory access for loads, stores andsends are issued in stage e0. Note that ‘send’ is an instruction toaccess data from memory and transmit it from the tile. Stages e1, e2 ande3 are referred to as to the memory stages. Accesses to the memory arepipelined over four stages, an input stage, a RAM macro, an output stagefor the RAM macro and a final output stage. However, the pipeline depthis referred to three stages since it does not introduce logic before thefirst stage (taking input from e0) or after the last stage (providingoutput to e4). In stage e4 of the pipeline there is a memory paritycheck and loads are picked up. All execution is speculative until stagee5 where, in the absence of exceptions, state updates are committed backto the context register files. There is one exception to this: storesare issued in stage e0 and cannot subsequently be prevented fromcommitting to memory. This causes a complication when handlingexceptions.

A further complication occurs because both the execution pipeline andthe receive logic can write to the memory.

Depending on when exception events are detected, it may not be possibleto prevent all architectural state updates associated with the cause.Exception events are therefore categorised with the following differentlevels of precision:

-   -   precise exception events are those where state is consistent        with the instruction pre-commit.    -   Semi-precise exception events are those where the state is        consistent with the instruction post-commit. Semi-precise        exception events apply to store instructions only.    -   Super imprecise exceptions are those where the state is        consistent with instruction post-commit, but not in the context        of store instructions.

The present disclosure addresses the relationship between exceptionswhich are raised by the receive logic at receive interface 17 and theprocessing of instructions being executed. In particular the presentdisclosure addresses super imprecise events. The purpose of a retirementexception is to propagate an exception event that is asynchronous withrespect to the instruction stream of a context, into that context toprevent it from further execution.

Reference is made to FIG. 4 to describe the example operation of theexecution pipeline on the detection of retirement exception events. Atthat time t it is assumed that a retirement exception event was logged.For the purposes of explanation, a retirement exception event 400 isshown at time 0 and the exception is logged at time 0. Note that inexamples, the exception event may be a tile exception event detected bythe execution pipeline or something else in the tile, or a receiveexception which had been logged in a final stage of the receivepipeline.

At time t, a next retiring instruction INS is received. The receiveexception has been logged (at time 0). so if an instruction was toretire in cycle t+5 (i.e. in its e5 stage) then it would raise aretirement exception Ex. At that point the exception Ex is raised, and asignal is sent to prevent further instruction issue into that context.FIG. 4 shows at t+6, instruction issue is prevented (illustrated by thedashed boxes) due to the exception raised at t+5. In other words, thedashed box represents a state where that particular context is nowexcepted because the exception has been raised, i.e. instruction issueis prevented. Note that this mechanism allows for up to five stores tocommit to memory after the exception event occurs. This is shown by thegrayed out boxes in FIG. 4, representing five exemplary potential superimprecise stores.

Exception events are logged by the receive logic at receive interface 17when they occur. A tile instruction raises a receive exception when itretires and sees that an exception has been logged by the receive logic.Reference is made to FIG. 9A which shows an example in general terms thesequence of events in receive exceptions described more fully herein. Atstep S10 it is detected than an exception event has occurred. Exceptionevents which are relevant herein are those which can occurasynchronously to the instruction stream. Some examples are given later.At step S20 the exception event is logged. The timing at which theexception event is logged is discussed later with reference to thereceive logic pipeline. The exception may be logged by writing it into acontext register file for holding exception status for asynchronousevents, such as those arising from the receive logic. When aninstruction retires, part of the instruction retirement process is tocheck the exception status register for asynchronous events, at stepS30. If it detects that an exception event has been logged, theinstruction raises an exception at its retirement S40. The timing atwhich this is done with respect to the execution pipeline is discussedlater.

FIG. 9B shows an example alternative flow which concerns therelationship between exceptions which are raised by the executionpipeline of the tile and the operation of the receive logic at thereceive interface 17. At step S21 a tile exception event is logged at afinal stage in the execution pipeline. A step S31 each time data isreceived, the receive logic checks whether or not a tile exception eventhas been logged. If it has, the receive logic ceases to receive datafrom that point. However, the exception is not logged until a finalstage of the receive pipeline at step S41. This is discussed in moredetail later.

The following are examples of exceptions corresponding to events thatcan occur asynchronously to the invention thread execution. Each carriesa particular exception type to be indicated in an exception register.

-   -   memory parity error, causes exception type MERR (logged by        memory stages of the fetch unit and execution pipeline, raised        at retirement);    -   exchange receive error causes exception type ERERR (logged by        receive logic, raised at retirement);    -   exchange parity error causes exception type EERR (logged by        receive logic, raised at retirement);    -   retirement break causes exception type TEX CPT_RB RK (a debug        retirement exception raised by the tile instruction).

For each of the above exceptions, the event is logged by writing one ofthe context registers which may then be checked at retirement of aninstruction. According to the present disclosure, retiring instructionsare permitted to commit to memory before they raise a retirementexception.

Reference will now be made to FIG. 3 to describe an example of thereceive logic at the receive interface 17. The receive logic is apipeline comprising eight stages which are labelled x0 . . . x7 in FIG.3. Note that x1 is an optional delay stage which is used in certaincircumstances which are not outlined here. The received pipeline isaligned with the execution pipeline 13 by the point at which accessesare issued to the memory. With a load/store-receive conflict, a receiveexception will correspond to the tile processor instruction thatperformed the memory access. As such, stage x2 corresponds to stage e0,and stage x7 corresponds to e5, the instruction commit point. ContextStatus Register (CSR) writeback is carried out in stage x7. Stage x2initiates memory access via a memory access unit. Before describing thereceive pipeline, reference is made to FIG. 10 to explain access to thememory 11.

FIG. 10 is a schematic view of the memory 11 which shows its accessports and the execution pipeline 18 and receive pipeline 17. FIG. 10illustrates the tile memory 11. The tile memory may contain multiplebanks (Bank 0 . . . Bank m) of RAM macros, for example each RAM macromay have a capacity of 16 KB. Multiple sets of banks may be provided insome embodiments. Each tile memory has three access ports. Port D0 is aread/write port. Port D1 and port D2 are read only. Writes to the memorymay come from one of two places. The loadstore unit LSU which operatesat stage s e0 to e5 of the tile execution pipeline 18 commits to memorythe store which was issued in stage e0. Stores to memory which are madeby the execution pipeline 18 are a result of instructions being executedearlier in the execution pipeline, as described earlier. The receivepipeline 17 of an EXIN unit (shown in FIG. 10) operates to store datareceived from outside the tile.

The example receive pipeline illustrated in FIG. 3 operates to receivedata (exchange data 300) incoming to the tile. The seven stages ofoperation of the exchange receive pipeline are shown in FIG. 3. In thefirst stage x0, data 300 is received. Ignoring for now the stage x1because it is not pertinent to the present disclosure, at the next stagex2, a store is issued to the memory. Stages x3, x4 and x5 represent tilememory stages. These correspond to the tile memory stages of theexecution pipeline, and are as described above with reference to theoperation of the memory. Stage x6 is a delay stage, which can be neededin some embodiments for timing purposes Stage x7 represents the statecommit point where state is written to the context registers. At thispoint, the following states are written into exception registers whichform part of exception register 38 a, 38 b, see FIG. 7. They are named$CTXT_STS.EERR (38 a) and $CTXT_STS.ERERR (38 b). The EXIN unit (whichhis shown in FIG. 10) denotes the logic for committing the store tomemory. An access conflict within a specified range of memory may occurwhen two valid memory accesses target that range simultaneously. Forexample, if the load store unit LSU and the EXIN unit attempt to accessthe memory via port D0 at the same time, this constitutes a memoryaccess conflict. A multiplexor 220 is connected to receive potentialaccesses from the LSU unit and the EXIN unit to permit either the LSUunit or the EXIN unit to access the memory. The output of themultiplexor 220 is connected to the port D0. Note that port D2 is a readonly port connected to the load/store unit. Port D1 is a read only portwhich can be read by the load/store unit or by an external exchange unitfor transmitting data out of the tile responsive to a ‘send’instruction. This is not germane to the present invention and thereforeis not discussed further herein. To resolve conflicts, in general, theread/write port (D0) has priority over the read only ports (D1, D2)since this provides consistent behaviour for all store exceptions, whichare classed as super-imprecise and define to always update the memorycontents. For a conflict between a load operation at the load/store unitand the EXIN unit, the EXIN unit takes priority and stores to memory. Inthe case of a conflict between the load/store unit operating a store,and the EXIN unit operating store, the EXIN unit takes priority andstores to memory. This is performed under the control of the multiplexor220 The multiplexer 220 is controlled by the receive logic usingaddresses stored in the context status register. In the case of anaccess conflict between an instruction being executed by the executionpipeline seeking to store data to memory, and the receive logic seekingto store data to memory, the receive logic logs the exception but theinstruction will raise an exception before retirement (because it hasdetected the conflict) so the final state is an exception emanating fromthe instruction, with the receive exception logged, but not raised.

With a load store receive conflict, the receive exception willcorrespond to the tile processor instruction that performed the memoryaccess. As such, stage x2 of the exchange receive pipeline 17corresponds to stage e0 of the execution pipeline 18. Stage x7 of theexchange receive pipeline 17 corresponds to stage e5 of the executionpipeline 18, which is the instruction commit point.

The updates to the exception register status $CTXT_STS.ERERR and$CTXT_STS.EERR are performed at stage x7 of the receive exchangepipeline, and are consistent with the commit point of the tile executionpipeline.

The receive interface 17 logs exceptions via writes to the exceptionregister 38 a, 38 b $CTXT_STS.ERERR or $CTXT_STS.EERR. Exchange receiveand parity errors are raised independently, so it is possible that thesecan be set in the same cycle.

The exception behaviour of the receive logic is consistent with thestate of the tile processor by delaying any effects of receiveexceptions to the tiles e5 commit point. This ensures:

-   -   1. That any receive exception event that is dependent on the        state of the tile is raised at the commit point of the        instructions that it corresponds to. Note that if the exception        was logged at some earlier point, it might appear to be raised        in the past of the corresponding instruction.    -   2. That any speculative effect of an instruction, (such as a        read from memory) does not affect the receive exception        behaviour until that speculation is confirmed at the e5 commit        point.

Even though writes to memory are squashed in stage x0 of the receivepipeline logic, due to timing constraints, the effect of delayingreceive exceptions to stage x7 is that any data received in a subsequentsix cycle window (pipeline stages x2 to x7) is written to memory.Therefore, in the worst case, after a receive exception occurs, sevendata words are written to memory: the one responsible for the exceptionand six more following it. This behaviour is consistent for all receiveexceptions.

The receive logic at the receive interface 17 starts dropping data afterit has detected an exception event if one of the following occurs:

-   -   a) a tile processor exception has been raised,    -   b) any receive error has been logged,    -   c) an exchange parity error has been logged.

In one embodiment, only the first receive exception may be logged.

The timing of imprecise stores is the same as that for receiveexceptions. After the tile exception is committed, up to the pipelinedepth, from stages x2 to x7, of stores can commit to memory. When areceive exception occurs, it causes the next tile processor instructionthat completes its execution to raise an exception upon retirement. Thereceive exception causes a write to $CTXT_STS.ERERR/EER in stage x7 andin the following cycle the receive exception event is signalled to thetile processor by the value in that register. This can be seen fromFIGS. 5 and 6 which illustrate an example of the exception timing.Reference will now be made to FIG. 5.

When a receive exception occurs, there is a window of six cycles beforereceive data will be dropped. This is due to the depth of the receivepipeline between the memory interface in the exception logic. FIG. 5illustrates this timing with data arriving on each cycle. For thepurposes of explanation only, data words DW1 . . . DW8 are shownreceived in sequence. DW1 is received in cycle t-8, DW2 in cycle t-7,etc. Two excepted data words DW9 and DW10 are also shown (describedbelow).

At time t-8, a data word DW1 is received into the pipeline at stage x0.The receive pipeline operates six stages x2 to x7 as described earlierto time t-2. A subsequent data word DW2 is received into stage x0 attimet-7, and the receive logic pipeline operates its stages from x2 tox7 up to time t-1. An exception is encountered in the receive interface17 when the subsequent data word DW2 is received. The receive exceptionEx is committed (by writing $CTXT_STS, ERERR or EERR to register 38 a or38 b) in stage x7 of the second data DW2 word that was received andprocessed (at time t). Note that by this time, up to six subsequent datawords (DW3, DW4 . . . DW8) may have been received and processed by thepipeline up to the point of being stored into memory. These up to sixother data words may be referred to as “imprecise stores”. At the pointat which the receive exception Ex is committed (at t-1), the receivelogic can start dropping data words, including DW9 and DW10 shown inFIG. 5 by dashed boxes. The instruction running on the tile processorcan retire at the execution stage e5. On retirement, it reads thecontext state register, detects the exception and signals an exceptionto be indicated externally of the tile. Note that this is the earliestpoint it can occur, because it needs to read the exception from theregisters to which the receive exception was written.

Reference will now be made to FIG. 6 to explain the timing of processorexceptions. Processor exceptions can cause the receive logic to stop,but as is the case with receive exceptions there is a latency for thisto take effect. When the receive logic detects that a tile processorexception has been logged and data arrives, it signals the write tomemory by writing an exception value TEXP_RERR_MODI to $CTXT_STS.ERERR,after any of the receive data already in the pipeline has beencommitted. The exception value TEXP_RERR_MODI can be used for debugpurposes. As illustrated in the example of FIG. 6, for the purposes ofexplanation only, data words DW1 . . . DW7 are shown received insequence. DW1 is received in cycle t-7, DW2 in cycle t-6, etc. Twoexcepted data words DW8 and DW9 are also shown (described below).

The first data word DW1 is received at time t-7 and processed by thereceive pipeline. An exception is raised at stage e5 of the processorpipeline which is executing an instruction, and the exception iscommitted by writing to registers 38 c of the context status registers$CTXT_STS at time t-1, for example, one or more of bits SU, W1, W2-W7may be written. Subsequent data words DW2-DW7 continue to be receivedand stored. These may be referred to as “imprecise stores”.

At time t, a data word DW8 may be received in stage x0 of the receivelogic but this data word is dropped at that stage, by not signalling awrite to memory. Subsequent incoming data words (e.g. data word DW9) arealso dropped. In contrast to a receive exception, data words are droppedfrom stage x0, the first stage of the receive pipeline. A receiveexception corresponding to the tile exception is not logged at stage x0in that case. Instead it is logged at stage x7 (at time t+6). Asmentioned above, in the meantime incoming data words DW2 to DW7(referred to herein as imprecise stores) corresponding to the exceptionwhich have already been received are processed in the receive logicpipeline, including being stored in memory. Data words received aftertime t are to be dropped as a result of the exception raised by theexecution pipeline. Two examples of such dropped data words, DW8 and DW9are shown in faded rectangles in FIG. 6.

Reference will now be made to FIG. 7 to illustrate the contextregisters. FIG. 7 shows an example set of context registers which can beused to log exceptions as described above. In some embodiments, theseregisters may be associated with the supervisor thread which manages theallocation of threads in the tile. However, any set of context registersmay be utilised. As described above, a set of context registers isavailable to the receive logic for logging exception events which occurat the receive logic. These are labelled for example 38 a and 38 b. Forthe sake of completeness, the context registers are also shown toinclude program count PC registers and status registers SR which holdstatus of the thread while it is operating.

In addition, the context registers include register 38 c which can holdexception events relating to operation of the execution pipeline 18. Forexample, these can include bit SU, W1, W2 . . . W7, each representing astatus for each context. Each status can indicate the presence of anexception of any type.

In the above description, the timing of exception handling and thesetting of exception raised status is discussed. There are twosituations in which exceptions can arise. One group of exceptions ariseduring normal operation (that is, intended normal operation which issomehow interrupted due to a fault), and another set of exceptions arisein the context of debugging. As discussed above, exceptions indicateunusual and potentially problematic execution conditions, that divergefrom the normal execution behavioural program. When in a debug mode,exceptions are deliberately created in order to allow debugging processto be carried out. Whether or not the chip is in a debug mode, it isimportant to be able to ascertain the cause of exceptions.

As discussed above, receive exceptions are logged in the contextregisters 38. This state can be inspected at retirement of aninstruction to determine if an exception should be raised. The contextregisters may indicate different types of errors. For example, a memoryerror flag may be asserted if the particular exception relates to aparity or error check in the memory. The exception register status foreach context may indicate the type of the exception (e-type), inaddition to an indication as to whether or not an exception should beraised outside the processing unit. Conversely, each time data isreceived by the receive logic at the receive interface 17, the contextstatus bits SU, W1 . . . W7 in register 38 c are checked to see if theexecution pipeline of the tile has raised an exception, to determine ifthe receive logic should stop receiving data.

FIG. 8 illustrates schematically the architecture of the computercomprising a plurality of tiles 4. Each tile may have an inspectioninterface such as a debug interface which allows access to the tilethrough a computer debug interface on the computer. A debug tool canaccess any tile via this computer debug Interface. Each tile can issuean exception raised signal. This is raised on a wire or set of wireswhich is connected from each individual tile to the debug interface.There, a logic circuit which function as an OR gate receives all of theexception raised wires and generates a global exception raised signal,the state of which depends on the state of the inputs to the logic. Thelogic is such that while any exception raised signal from an individualtile is active, the global exception raised signal will be active. Thecomputer in the example of FIG. 8 may be provided on a single die. Inaddition to the array of multiple processor tiles 4, there is an on-chipinterconnect 34 which allows data to be exchanged between the tiles.These are referred to as internal exchanges.

Processing and internal exchanges may be carried out in a timedeterministic manner. That is, the execution of the instructions in thepipeline has a timing determined by a compiler which generates theprogram for each tile. In addition, internal data exchanges within thechip between tiles on the chip are also time deterministic. That is, thetime at which data is to be transmitted from a transmitting tile to arecipient tile is known relative to the time at which the recipient tilereceives that data. Nevertheless, the time at which the receive pipelinereceives data relative to the operation of the execution pipeline on thetile is not known, and is considered to be asynchronous. Moreover, dataincoming from externally of the chip is asynchronous A tile does notknow when it might receive a piece of data which has come in externallyof the chip through an external exchange mechanism. The chip 2 may beimplemented alone on its own single chip integrated circuit package, oris one of multiple dies packaged in the same IC package. The on-chipinterconnect may also be referred to herein as the exchange fabric 34 asit enables the tiles 4 to exchange data with one another. Each tilecomprises a respective instance of the execution unit and memorydescribed above. Each chip 2 may also comprise one or more externallinks 8 enabling the chip 2 to be connected to one or more externalprocesses on different chips. These external links may comprise any oneor more of: one or more chip to host links connecting the chip to a hostprocessor, and/or one or more chip to chip links for connecting togetherwith one or more other instances of the chip 2 on the same IC package orcard, or on different card. Data received into the chip via the one ormore external links is handled by the receive logic on each tile, asdiscussed above. Note that this data may be received asynchronously tothe internal operation of each tile.

While particular embodiments have been described, other applications andvariants of the disclosed techniques may become apparent to a personskilled in the art once given the disclosure herein. The scope of thepresent disclosure is not limited by the described embodiments but onlyby the accompanying claims.

The invention claimed is:
 1. A processor comprising: memory; anexecution pipeline having a plurality of pipeline stages configured toprocess data provided to the execution pipeline and to store a result ofthe processing into the memory; and a receive pipeline having aplurality of pipeline stages configured to handle incoming data to theprocessor and storing the incoming data into memory; context statusstorage configured to hold an exception indicator of an exceptionencountered by the execution pipeline while the execution pipelineprocesses data; wherein the receive pipeline is configured to determinethat an exception has been committed to the context status storage bythe execution pipeline, to suppress a write to memory of any incomingdata to be handled by the receive pipeline and to commit a correspondingexception indicator to the context status storage at a final one of itspipeline stages.
 2. The processor of claim 1, wherein the executionpipeline is configured to check the context status storage when retiringan instruction and on detection of the corresponding exception indicatorin the context status storage to cause an exception signal to be raised.3. The processor of claim 1, wherein the receive pipeline is configuredto process a first incoming data word in sequential pipeline stages, andto process one or more subsequent incoming data words in successive onesof the pipeline stages, whereby multiple pipeline stages are configuredto operate on successive data words at the same time.
 4. The processorof claim 1, wherein the pipeline stages of the receive pipeline aretimed according to a clock signal.
 5. The processor of claim 4, whereinthe pipeline stages of the execution pipeline are timed according to theclock signal.
 6. The processor of claim 1, wherein the receive pipelineis configured to determine in a first one of the pipeline stages thatthe exception has been committed.
 7. The processor of claim 1, whereinthe execution pipeline is configured to process in successive pipelinestages incoming data comprising data words, and to carry out a resultstoring operation for each incoming data word, wherein a number ofresult operations available to store any result to memory betweendetection of the exception and commitment of the exception indicatorcorrespond to one less than a number of pipeline stages in the executionpipeline.
 8. The processor of claim 1, wherein the execution pipelineand the receive pipeline operate contemporaneously, the processorfurther comprising access logic to resolve memory write conflicts ofdata from the execution pipeline and the receive pipeline.
 9. Theprocessor of claim 8, wherein the access logic is configured toprioritize a memory write of data from the receive pipeline over datafrom the execution pipeline in response to a memory access conflict. 10.The processor of claim 1, which is configured to set an error value inresponse to detecting a write to memory after an exception has beencommitted.
 11. A method of handling exceptions in a processor, themethod comprising: handling incoming data, by a receive pipeline havinga first plurality of pipeline stages, and storing the incoming data intomemory of the processor; executing an operation on data, by an executionpipeline having a second plurality of pipeline stages, and storing aresult of the operation into the memory of the processor; determining bythe receive pipeline that an exception has been committed by theexecution pipeline; suppressing a write to the memory of any incomingdata to be handled by the receive pipeline; and committing an exceptionindicator to a context status storage at a final one of the pipelinestages of the receive pipeline.
 12. The method of claim 11, furthercomprising: checking the context status storage when retiring aninstruction; and causing an exception signal to be raised on detectionof the exception indicator in the context status storage.
 13. The methodof claim 11, further comprising: processing a first incoming data wordin sequential pipeline stages, and processing subsequent incoming datawords in successive ones of the pipeline stages, whereby multiplepipeline stages of the first plurality of pipeline stages are operatingon successive data words at the same time.
 14. The method of claim 11,further comprising: timing the pipeline stages of the receive pipelineand the execution pipeline according to a clock signal.
 15. The methodof claim 11, further comprising: detecting the exception indicator in afirst pipeline stage of the first plurality of pipeline stages.
 16. Themethod of claim 15, wherein data processed by the execution pipelinecomprises successive data words, and wherein a number of the data wordswhich are stored into memory between detection of the exception by theexecution pipeline and commitment of the exception indicator correspondto one less than a number of pipeline stages in the execution pipeline.17. The method of claim 11, wherein the execution pipeline and thereceive pipeline operate contemporaneously, the method furthercomprising: resolving memory write conflicts of data from the executionpipeline and the receive pipeline.
 18. The method of claim 17, furthercomprising: prioritizing a memory write of data from the receivepipeline over data from the execution pipeline in response to a memoryaccess conflict.
 19. A method comprising: storing incoming data intomemory of a processor by a receive pipeline having a first plurality ofpipeline stages, wherein the processor further includes an executionpipeline having a second plurality of pipeline stages; committing anexception by the execution pipeline; detecting the exception by thereceive pipeline; suppressing a write to the memory of any incoming datato be handled by the receive pipeline; and committing an exceptionindicator to a context status storage at a final one of the pipelinestages of the receive pipeline.
 20. The method of claim 19, furthercomprising: checking the context status storage when retiring aninstruction; and causing an exception signal to be raised on detectionof the exception indicator in the context status storage.
 21. The methodof claim 19, further comprising: processing a first incoming data wordin sequential pipeline stages, and processing subsequent incoming datawords in successive ones of the pipeline stages, whereby multiplepipeline stages of the first plurality of pipeline stages are operatingon successive data words at the same time.
 22. The method of claim 19,further comprising: timing the pipeline stages of the receive pipelineand the execution pipeline according to a clock signal.
 23. The methodof claim 19, wherein data processed by the execution pipeline comprisessuccessive data words, and wherein a number of the data words which arestored into memory between detection of the exception and commitment ofthe exception indicator correspond to one less than a number of pipelinestages in the execution pipeline.
 24. The method of claim 19, whereinthe execution pipeline and the receive pipeline operatecontemporaneously, the method further comprising: resolving memory writeconflicts of data from the execution pipeline and the receive pipeline.25. The method of claim 19, further comprising: prioritizing a memorywrite of data from the receive pipeline over data from the executionpipeline in response to a memory access conflict.